1. Field of the Invention
The present invention relates to a piezoelectric resonator, and more particularly, to a piezoelectric element for use as an oscillator in a communication filter and a clock generator.
2. Description of the Related Art
As one of conventional piezoelectric elements, known is a piezoelectric element produced by polarizing a piezoelectric ceramic body having a layered perovskite structure perpendicular to the orientation axis of the piezoelectric ceramic body. In such a piezoelectric element, electrodes are formed on both of the end faces of the piezoelectric ceramic body. Piezoelectric vibration can be excited by applying an electric field in the polarization direction.
Moreover, the inventors of the present invention have proposed a piezoelectric element of such a type that interdigital electrodes are formed on the front and back main faces of a single plate piezoelectric ceramic body for excitation. This piezoelectric element does not use a lamination process with a large number of steps. Accordingly, the piezoelectric element can be utilized conveniently, industrially, and has a high reliability with no problem on the optimization of coupling of an electrode interface to a ceramic.
Referring to the piezoelectric element produced by forming electrodes on both of the end faces of a piezoelectric ceramic body having a layered perovskite structure, and polarizing the ceramic in the direction perpendicular to the orientation axis, it is necessary for the piezoelectric element to have a sheet or rod shapes structure elongating in the electric field application direction. Such a structure has the problem that the electrostatic capacitance is low and the impedance is high, so that it is difficult to achieve impedance matching to a circuit.
Moreover, as regards the piezoelectric element in which the interdigital electrode is formed on a main face of a single plate piezoelectric ceramic body, e.g., the back main face thereof, it is necessary to reduce the thickness of the element in order to polarize the whole of the element when the piezoelectric element is used at a high frequency. This causes the problem that the mechanical reliability of the piezoelectric element can not be enhanced when the element is used at a high frequency.